The present invention relates to a polymer electrolyte fuel cell, particularly to a method of producing an electrode catalyst particle constituting a catalyst layer of an electrode of the polymer electrolyte fuel cell.
A fuel cell employing polymer electrolyte generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air. This fuel cell is basically composed of a polymer electrolyte membrane for selectively transporting hydrogen ions, and a pair of electrodes respectively formed on both sides of the polymer electrolyte membrane. The electrode is constituted by a catalyst layer composed of an electrically conductive carbon powder carrying a platinum group metal catalyst and a hydrogen ion conductive polymer electrolyte mixed therewith, and a gas diffusion layer made of, for example, a water repellent treated carbon paper, which has both gas permeability and electronic conductivity and is formed on the outer surface of the catalyst layer.
Gas sealing members or gaskets are arranged on the periphery of the electrodes with the polymer electrolyte membrane disposed therebetween so as to prevent a fuel gas and an oxidant gas supplied to the electrodes from leaking out or prevent the two types of gases from mixing with each other. The gas sealing members or gaskets are combined integrally with the electrodes and polymer electrolyte membrane beforehand. This is called “MEA” (membrane electrode assembly). Disposed outside the MEA are conductive separator plates for mechanically securing the MEA and for connecting adjacent MEAs electrically in series. At a portion thereof to come in contact with the MEA, the separator plates have gas flow channels for supplying reactant gases to the electrodes and for removing a generated gas and excess gas. While the gas flow channels may be provided separately from the separator plates, grooves are usually formed on the surfaces of the separator plates to serve as the gas flow channels.
In general, the gas diffusion layer of the electrode of the polymer electrolyte fuel cell is composed of a porous carbon layer of a water repellent treated nonwoven carbon fabric or the like. In some cases, for the purpose of keeping the catalyst layer or the polymer electrolyte membrane humidified, a water repellent carbon layer is provided between the interface of the catalyst layer and the gas diffusion layer. Generally, the water repellent carbon layer is formed in the following manner.
First, carbon particles are mixed with a dispersion of polytetrafluoroethylene fine particles containing a surfactant, and the whole is subjected to a process such as drying or filtration, to give a mixture of carbon particles and polytetrafluoroethylene fine particles. Next, water or an organic solvent is added to the mixture to produce an ink. The ink thus produced is applied, by a process such as screen printing, spray coating, doctor blading or roll coating, onto one surface of a nonwoven carbon fabric which serves as the gas diffusion layer, followed by baking at a temperature from about 300° C. to about 400° C. to burn off the surfactant. In general, the water repellent carbon layer is formed on the gas diffusion layer in this manner. Herein, the gas diffusion layer is combined with the catalyst layer such that the water repellent carbon layer is in contact with the catalyst layer.
Meanwhile, the catalyst layer is usually composed of a thin coating film of a mixture of an electrically conductive carbon powder carrying a platinum group metal catalyst and a hydrogen ion conductive polymer electrolyte. At present, perfluorocarbon sulfonic acid is commonly used as the hydrogen ion conductive polymer electrolyte. The catalyst layer is formed in the following manner. An electrically conductive carbon powder carrying a catalyst such as platinum is mixed with a solution or dispersion of a polymer electrolyte, which is prepared by dissolving or dispersing a polymer electrolyte in an alcohol solvent such as ethanol. The mixture thus obtained is added with an organic solvent having a relatively high boiling point, such as isopropyl alcohol or butyl alcohol, to produce an ink. The ink is applied to a polymer electrolyte membrane or other substrate by a process such as screen printing, spray coating, doctor blading or roll coating, thereby forming a catalyst layer. Further, a polytetrafluoroethylene fine powder or a solution or dispersion thereof was added to the above-described ink in order to retain, in the vicinity of the reaction site in the catalyst layer, product water and water contained in the supplied gas, while discharging excess water to the outside.
Commercially available solutions or dispersions of a hydrogen ion conductive polymer electrolyte have a concentration of approximately 10%. Thus, in order to mix a hydrogen ion conductive polymer electrolyte with an electrically conductive carbon powder carrying a catalyst, a solution or dispersion of a polymer electrolyte containing a large amount of a solvent or dispersion medium has to be mixed with the electrically conductive carbon powder carrying a catalyst. This results in a reduced viscosity of the resultant ink, making it impossible to produce an ink having a sufficiently high viscosity required for a process such as screen printing. To solve this problem, a method has been employed, in which a solvent of an ink is evaporated to produce a high viscosity ink. However, it is difficult to prepare homogeneous ink by this method. Therefore, another method has also been employed, which involves: previously evaporating and solidifying a solution or dispersion of a hydrogen ion conductive polymer electrolyte; redissolving or dispersing it in an organic solvent or dispersion medium having a relatively high boiling point, such as isopropyl alcohol and butyl alcohol, to prepare a solution or dispersion containing a hydrogen ion conductive polymer electrolyte at a desired concentration; and producing an ink by using the solution or dispersion.
To achieve the practical utilization of the fuel cells, a further improvement in the power generating efficiency is required. To this end, it is important to attach, as uniformly as possible, a relatively thin layer of a hydrogen ion conductive polymer electrolyte on the surface of an electrically conductive carbon powder carrying a catalyst in the catalyst layer. Further, it is also important to attach such layer of a hydrogen ion conductive polymer electrolyte on as much catalyst as possible. At the same time, it is necessary to prevent a water repellent material, which is added into the catalyst layer for the purpose of water management, from coating the surface of a metal catalyst to reduce the reaction site.
However, since the conventional methods employ a catalyst ink prepared by mixing a catalyst powder, a solution or dispersion of a hydrogen ion conductive polymer electrolyte and a dispersion of a water repellent material, they have had a problem that the water repellent material adheres to the surface of the catalyst metal such as platinum to impede the supply of the reactant gases, thereby degrading the electrode performance. If the amount of the water repellent material to be added is decreased so as not to impede the supply of the reactant gases, a sufficient amount of the water repellent material required for water management cannot be added. The above-described reasons have posed a problem that a sufficient power generating property required for the practical utilization cannot be achieved for the fuel cells.